Science Demonstrations

User Tools

Site Tools

demonstrations:inverse_square_law_of_radiation

Inverse-Square Law of Radiation

Materials: ★★★ Requires materials not commonly found in school laboratories
Difficulty: ★★☆ Can be done by science teachers
Safety: ★★★ Only to be attempted with adequate safety procedures and trained staff

Categories: Nuclear Physics

Alternative titles: Geiger Counter Inverse-Square Demonstration

Summary

By measuring the number of radiation counts with a Geiger counter at different distances from a source, students can demonstrate the inverse-square law: radiation intensity decreases with the square of the distance from the source.

Procedure

  1. Place a radioactive source near a Geiger counter detector at the smallest safe distance (e.g., 20 mm).
  2. Record the count rate for a set period of time (e.g., 15 seconds).
  3. Repeat the measurement several times to find an average count rate.
  4. Move the source further away (e.g., in steps up to 90 mm) and record counts again at each distance.
  5. Plot a graph of count rate versus distance.
  6. Compare how the measured values decrease as the source is moved further from the detector.
  7. Discuss how the count rate relates to the inverse-square law: doubling the distance reduces the counts to about one quarter.

INVERSE-SQUARE LAW - A-level Physics Required Practical - Science Shorts:


📄 Exploring the Intensity of Radiation - farLabs: https://www.farlabs.edu.au/nuclear/explore-inverse-square-law/

Variations

  • Test different radioactive sources (alpha, beta, gamma) to see if distance affects them differently.
  • Measure at smaller or larger distances to refine the curve.
  • Add shielding materials (paper, aluminum, lead) to compare how absorption interacts with the distance effect.
  • Use the same method with a non-radioactive source of waves (e.g., light or sound) to show that inverse-square applies generally.

Safety Precautions

  • Handle radioactive sources carefully and keep exposure time short.
  • Use only approved, low-activity classroom sources.
  • Wash hands after handling radioactive materials.
  • Keep food and drink away from the work area.
  • Always return sources to their shielded storage when not in use.

Questions to Consider

  • What does the shape of your graph suggest about how radiation spreads? (It shows that intensity decreases according to the inverse-square law.)
  • If you double the distance, what happens to the count rate? (It falls to about one quarter.)
  • Would the graph shape be different for alpha, beta, or gamma sources? (The overall inverse-square relationship remains, but alpha radiation may drop off faster because it is easily absorbed by air.)
  • Why does radiation decrease with distance in this way? (As particles or photons spread out in all directions, their intensity is diluted over the surface area of a growing sphere.)